1. Field of the Invention
The invention relates to semiconductor wafer processing systems and, more particularly, the invention relates to a method and apparatus for conditioning an electrostatic chuck used to retain a semiconductor wafer after a maintenance cycle.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer processing system. Although electrostatic chucks vary in design, they all are based on the principal of applying a voltage to one or more electrodes embedded in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece.
A problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. Another conventional solution, which purportedly removes the charge more quickly, is to reverse the polarity of DC voltage applied to the electrodes. This technique is described in the context of a chuck having two electrodes (a bipolar chuck) in U.S. Pat. No. 5,117,121 issued May 26, 1992 to Watanabe, et al.
A shortcoming that has been observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic force remains between the workpiece and the chuck. This residual electrostatic force necessitates the use of a large mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck unpredictably into a position from which it is difficult to retrieve using a conventional wafer transport robot.
This accumulation of the residual charge is also detrimental to chucking of a subsequent wafer. The accumulated charge interferes with the chucking voltage by either being additive, if the accumulated charge has the same polarity as the chucking voltage, or is subtractive, if the accumulated charge has an opposite polarity as the chucking voltage.
To decrease the residual charge, the chuck can be utilized at high temperatures. In certain ceramic chucks, using the chuck at a higher temperature (e.g., greater than 200xc2x0 C.) makes the chuck material more conductive. As such, some of the residual charge will dissipate by conduction through the chuck to the electrodes as long as the chuck is maintained at the high temperature. Additionally, a plasma cleaning step may be performed after wafer processing occurs. Typically, an inert gas (e.g., Argon) is introduced into a chamber containing the electrostatic chuck. The plasma is ionized thus forming a conductive path from the electrostatic chuck surface (containing the residual charges) and a grounded chamber component (e.g., a chamber wall or the like). The ions bombard the electrostatic chuck surface thereby dislodging the residual charges. The charges then move through the plasma to ground. Reactive species (e.g., oxygen or hydrogen may also be used). The plasma cleaning step also induces high temperatures in the electrostatic chucks to further dissipate the residual charges.
Unfortunately, the dissipating effect of the high temperature maintenance process (either by direct heating or plasma processing) is greatly reduced as the chuck begins to cool and is prepared for wafer processing. Therefore, there is a need in the art for a method to retain the effect of dissipating residual charges after the maintenance cycle ends (during a cooldown period).
The disadvantages of the prior art are overcome by the present invention of a method and apparatus for conditioning a residual charge from an electrostatic chuck. The conditioning process involves exposing the surface of an electrostatic chuck to a source of energy that bombards the surface with non-ionized radiation, e.g., photons or microwaves. The electrostatic chuck can be any form of chuck having one or more electrodes embedded into a dielectric body. Preferably, the dielectric body is fabricated from a material having a relatively low resistivity (i.e., the material is semiconductive) such as aluminum nitride. Such material contains donor atoms that can be excited to enhance conductivity of the material. In one embodiment of the invention, the conductivity of the chuck material is promoted by exposing the surface of the chuck to photons from a lamp or lamps. After exciting the material, the residual charge on the chuck surface is substantially reduced.
To discharge the electrostatic chuck in accordance with the present invention, bake out lamps located in the process chamber are turned on after each wafer is removed from the chamber and the surface of the chuck is exposed to illumination that discharges the surface.
Additionally, by using the illumination process of the present invention to remove the residual charge rather than relying on an electrostatic chuck being used at high temperatures only, electrostatic chucks that heretofore were expected to have a minimum temperature of 150 degrees C. can now be used at much lower temperatures, e.g., xe2x88x9240-+25 degrees C.